The American Cancer Society claims that there is a lifetime incidence of breast cancer in the United States of about one woman in eleven. A significant reduction in mortality would be gained by regular screening for all asymptomatic women, particularly for those most at risk. The long term objective of this research is the development of a device which is able to detect minimal cancers in the human breast using harmless doses of visible or near-infrared radiation. Because of small, but finite hazards associated with exposure to x-rays, a breast cancer screening method that uses non-ionizing radiation would be of considerable clinical benefit, particularly if it demonstrated a diagnostic performance equivalent to or better than x-ray mammography. Because of the overwhelming scatter of light through soft tissue, the spatial resolution performance of current breast transillumination methods is extremely poor. No light can penetrate a human breast without considerable scatter. However, it is proposed to investigate the merits and limitations of various schemes for producing high-resolution images using the small amount of transmitted radiation that is scattered through the breast along a path close to the straight line between the radiation source and the detector. The imaging method involves recording and discriminating between the times-of-flight of all transmitted photons, and using a fraction of the light with the shortest travel times to construct an image through the breast. The two major components of an imaging system are a picosecond pulse laser, and a streak camera. The photons within a single laser pulse incident upon the surface of a breast will be scattered throughout the tissue, and the streak camera measures the intensity of the transmitted light as a function of time. The photons detected first will have been deviated least from the optical axis. The intensity measured over some very small period of time after the photons first emerge from the tissue will be dependent upon the absorption properties of the tissue contained within some narrow volume element surrounding the optical axis. A single projection image may then be constructed by translating the optical axis in two dimensions over the surface of the breast. Preliminary experiments have dramatically illustrated the potential of time-of-flight imaging. The primary objectives are to develop and test a number of proposed imaging schemes, and to produce a prototype device capable of achieving single projection transmission images with a spatial resolution of a few millimeters or better. In addition, the same experimental arrangement will be used to explore methods of obtaining slice images through a breast using computed tomography.